Method for producing a hot strip

ABSTRACT

The invention relates to a method for producing a hot strip, in particular for producing a hot strip intended for the production of a cold strip with good deep-drawing characteristics; in which a steel melt comprising (in % by weight) C:≦0.07%, Si:≦0.5%, Mn:≦2.5%, Al:≦0.1%, N:≦0.01%, P:≦0.025, B:≦0.05, if need be up to a total of 0.35% of Nb, Ti and V, with the remainder being iron and the usual impurities is melted; in which the steel melt is continually output in one strand (S) from a permanent casting mould ( 1 ); in which the cast strand (S) immediately after discharge from the permanent casting mould ( 1 ) is led along a cooling line ( 2 ); in which the strand (S) is intensively cooled down to a temperature of A r1 ±25 K at a cooling rate (a LM ) of at least 3 K/s; in which, following its intensive cooling, the strand (S) is cooled by exposure to air for at least 30 seconds; and in which the strand (S) itself or thin slabs (D) divided off the strand (S) is/are reheated in a soaking furnace ( 5 ) before the strand (S) or the thin slabs (D) are hot rolled to form hot strip. The method according to the invention makes it possible, during processing of low-alloyed low-carbon steels, to reduce the required temperature in the soaking furnace such that the stress on the furnace is reduced without there being any reduction in the quality of the hot strip produced, or of the cold strip made from said hot strip.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing a hot strip, inparticular for producing a hot strip intended for the production of acold strip with good deep-drawing characteristics, made of a low-carbonlow alloyed steel, in which thin slabs are produced by continuous strandcasting; in which the strand emerging from a permanent casting mouldduring continuous strand casting passes along a cooling line; and inwhich the cast strand itself or thin slabs divided off the strand arereheated in a soaking furnace before they are hot rolled to form hotstrip.

In a known method of the above-mentioned type, which has become known asthe “CSP method”, thin slabs are divided off a steel strand produced ina continuous strand casting plant and after temperature equalisation ina tunnel furnace, are subjected to rolling in a multistand mill train soas to form hot strip. In the known method, as a rule the thin slabsenter the soaking furnace at a temperature between 950° C. and 1100° C.and are reheated therein to a temperature between 1100° C. and 1200° C.

By using the heat present in the strand after casting, the known methodmakes it possible to produce hot strip with reduced energy expenditurewhen compared to other conventional methods of this type. However, withthis method, the soaking furnace must be operated at very hightemperatures. Such high temperatures cause rapid wear of the furnace sothat the cost of furnace maintenance negates the benefit of energy gainsachieved. Nonetheless the high temperatures in the soaking furnace arerequired in the state of the art so as to keep in, or bring to,solution, the alloying constituents in the steel strand, saidconstituents during subsequent process steps in the production of thehot strip or of the cold strip made from said hot strip, causingprecipitation which has a decisive influence on the formation of aparticular microstructure in the finished hot strip or in the cold stripproduced therefrom.

From EP 0 686 702 A1 a modification of the above-mentioned method isknown in which the surface temperature of the slab between the permanentcasting mould and the soaking furnace is lowered in an adequate way tosuch an extent that a microstructural transformation from austenite toferrite/pearlite takes place in the slab. It is further stated that thetemperature attained during this process at a depth of 2 mm below theslab surface is preferably below 600° C.

It is the aim of the measures described in EP 0 686 702 A1 to achieve asituation in which even such slabs which as a result of the addition ofsubstantial quantities of secondary scrap to the melt containsignificant amounts of copper, enter the soaking furnace in a state bywhich excessive accumulation of copper in the region of the grainboundaries of the primary austenite is prevented. Otherwise suchaccumulation causes very considerable scale formation and in furtherstages of hot strip production can cause so-called soldering fractures.By cooling the slabs to a temperature below the A_(r3) temperature (thetemperature below which transformation of austenite to ferrite takesplace), a microstructure transformation with re-orientation of theaustenite grain boundaries is enforced during reheating in the soakingfurnace.

Slabs which are cooled in this way are reheated in the soaking furnaceto the high temperature to which the soaking furnace is usually set. Inorder to expend as little energy as possible for reheating, in themethod known from EP 0 686 702 A1, the depth of cooling and the timeintended for cooling are reduced to a minimum so that the temperature inthe interior of the slab is as high as possible when the slab enters thesoaking furnace.

Attempts to minimise both the wear of the soaking furnace and the energyexpenditure required for its operation, by reducing the furnacetemperature have shown that such a reduction in temperature inparticular during processing of low-alloyed low-carbon steels has anegative influence on the formation of precipitation in the subsequentprocess of hot strip production and cold strip production.

SUMMARY OF THE INVENTION

It is thus the object of the invention, in a process of the typementioned above, during processing of low-alloyed low-carbon steels toreduce the required temperature in the soaking furnace such that thestress on the furnace is reduced without there being any reduction inthe quality of the hot strip produced, or of the cold strip made fromsaid hot strip.

According to the invention this object is met by a method for producinga hot strip, in particular for producing a hot strip intended for theproduction of a cold strip with good deep-drawing characteristics; inwhich a steel melt comprising (in % by weight) C:≦0.07%, Si:≦0.5%,Mn:≦2.5%, Al:≦0.1%, N:≦0.01%, P:≦0.025, B:≦0.05, if need be up to atotal of 0.35% of Nb, Ti and V, with the remainder being iron and theusual impurities; in which the steel melt is continually output from apermanent casting mould; in which the cast strand immediately afterdischarging from the permanent casting mould is led along a coolingline; in which the strand is intensively cooled down to a temperature ofA_(r1)±25 K at a cooling rate of at least 3 K/s; in which, following itsintensive cooling, the strand is cooled by exposure to air for at least30 seconds; and in which the strand itself or thin slabs divided off thestrand is/are reheated in a soaking furnace before the strand or thethin slabs are hot rolled to form hot strip.

Since according to the invention the strand emerging from the permanentcasting mould is subjected to intensive cooling at cooling rates of atleast 3 K/s, during which cooling the strand is cooled to a temperatureof A_(r1)±25 K (the temperature at which the transformation fromaustenite to ferrite is completed), the precipitation required toachieve the desired material characteristics of the hot strip, is in atargeted way already introduced in the region in front of the soakingfurnace. Thus, during cooling by exposure to air, which cooling followsintensive cooling, there is sufficient time available for theprecipitation processes to be essentially complete at the time of entryinto the soaking furnace. At the same time, homogenisation of thetemperature in the strand takes place so that there is an eventemperature distribution at the time of entry into the furnace.

Since the formation of precipitation has essentially been completedprior to entry into the soaking furnace, the furnace temperature can belimited to a temperature which is below the reheating temperatureapplied in the conventional approach. Advantageously, the soakingfurnace temperature to be adhered to according to the invention is in arange whose lower limit is determined by the A_(r3) temperature andwhose upper limit is 1150° C.

A reheating temperature of max. 1050° C. is sufficient if the hot stripproduced according to the invention is used for the production of a coldstrip which after cold rolling is annealed in a continuous annealingfurnace. In this case, preferably no precipitation processes take placeduring any reheating connected with the production of the hot strip andcold strip, such process steps following reheating, so that it is nolonger necessary to bring to solution any alloying constituents whichtake part in the formation of precipitation.

By contrast, if from the hot strip produced according to the invention,a cold strip is rolled which after cold rolling is annealed in ahood-type annealing furnace, then the temperature in the soakingfurnace, during heating of the strand or the thin slabs, should be inthe range from 1100° C. to 1150° C. If the heating temperature exceeds1100° C., sufficient Al nitride will become soluble to produce adesirable “pancake” microstructure during hood-type annealing.

It has been found that a strip produced according to the invention has afine-grained microstructure which has a favourable effect on thedeep-drawability of a cold strip made from the hot strip. Thus inessence the invention provides a method which makes it possible toreduce the temperature in the soaking furnace, so that the service lifeof said soaking furnace is prolonged and the economy of the method, whencompared to the conventional approach, is improved. In addition, themethod according to the invention results in a product which isoutstandingly suitable for processing by way of deep-drawing.

Preferably several roll passes are made during hot rolling, wherein thefinish-rolled hot strip is 2 to 5 mm in thickness. In the last roll passa thickness reduction of ε_(h)>15% should be attained. The hot striprolled in this way has a particularly fine-grained microstructure whichfurther improves its deep-drawability. In this context “deformationε_(h)” refers to the ratio of thickness reduction during the last rollpass to the thickness of the strip at the time of entry into the lastroll stand of the hot-rolling mill train. Accordingly, the thickness ofa hot strip prior to the last-roll pass is for example h₀. Following thelast roll pass, the thickness of the strip is reduced to h₁. Accordingto the definition there is thus a deformation in the last roll pass ofε_(h) to (h₀−h₁)/h₀>15% where h₀=thickness of the hot strip at the timeof entry into the last roll stand, and h₁=thickness of the finish-rolledhot strip.

If hot rolling is to be carried out with the microstructure of the hotstrip being in the austenitic range, then the finish-roll temperature oncompletion of hot rolling is preferably at least 20° C. above the A_(r3)temperature. If by contrast, an essentially ferritic microstructure ofthe hot strip is desired after hot rolling, then it is advantageous ifthe finishing temperature on completion of hot rolling is below theA_(r1) temperature +50° C.

From the point of view of deep-drawing characteristics, a furtherimprovement of the microstructure of a cold strip made from the hotstrip according to the invention, can be achieved in that the totaldeformation ε_(ges), achieved during cold rolling of the hot strip, isat least 60%. In this context “total deformation ε_(ges)” refers to theratio of thickness reduction during cold rolling to the thickness of thenon-rolled strip at the time of entry into the cold-roll stand.According to this definition, the thickness of a hot strip producedaccording to the invention is for example h₀ after hot rolling. Aftercold rolling, the thickness of the strip has been reduced to h₁.According to the definition, there is thus a total deformation ofε_(ges) to (h₀−h₁)/h₀ where h₀=thickness of the hot strip at the time ofentry into the cold-roll stand, and h₁=thickness of the finish-rolledcold strip.

If, as mentioned above, the cold strip produced from the hot strip,after cold rolling is annealed in a continuous annealing furnace, thenthe finish-rolled hot strip should be coiled at a coiling temperature ofat least 650° C. Adherence to this minimum temperature promotes theformation of precipitation in the coiled hot strip so thatrecrystallisation of the cold strip can take place during continuousannealing, unhindered by precipitation.

By contrast, if a cold strip is produced which is to be annealed in ahood-type annealing furnace after cold rolling, then the finish-rolledhot strip should first be coiled at a coiling temperature of max 625° C.In this way the remainder of the alloying constituents which are stillpresent in the dissolved state and which take part in the formation ofprecipitation, are kept in solution. During hood-type annealing duringwhich the cold strip is exposed for an extended period to a temperaturewhich is lower than the temperature during continuous annealing,precipitation forms in the cold strip which precipitation is requiredfor the formation of the desired pancake microstructure in the coldstrip.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, the invention is explained in more detail by means of a drawingshowing one embodiment and by means diagrams. The following isdiagrammatically shown:

FIG. 1 a lateral view of the start of a production line for producing ahot strip from a cast steel strand;

FIG. 2 the gradient of A_(r1) and A_(r3) temperature depending on thecarbon content of a low-carbon steel; and

FIG. 3 the temperature gradient of the strand in the region of the startof a production line shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

A melt of a low-carbon low-alloyed steel is poured via a permanentcasting mould 1 to a steel strand S which measures between 20 and 70 mmin thickness.

Immediately after discharge from ther permanent casting mould 1, thesteel strand S, as it travels along a “metallurgical length” LM, isintensively cooled by cooling water which, from cooling devices 2arranged on both sides of the steel strand S, is directed towards saidsteel strand S. The cooling, rate a_(LM) achieved during the intensivecooling of the steel strand S within the metallurgical length LM is atleast 3 K/s, with the actually set cooling, rate a_(LM) depending on therespective thermal conductivity of the steel strand S and the requiredtemperature T_(LM) at the end of the metallurgical length LM. In anycase, the extent of intensive cooling is such that the temperatureT_(LM) of the steel strand S at the end of the metallurgical length LMis A_(r1)±25° C., for example 710° C. FIG. 2 shows the position of theA_(r1) temperature depending on the carbon content of the composition ofthe steel strand S.

Following the metallurgical length LM with the cooling devices 2positioned on said metallurgical length LM, the steel strand S passes ona roller table 3 along a cooling line LT in which cooling of the steelstrand S takes place by exposure to air. The steel strand S takes atleast 30 seconds to pass along the cooling line LT so that at the end ofthe cooling line LT the formation of precipitation in the steel strand Sis essentially complete and the temperature distribution is homogenous.After passing the cooling line LT, depending on the design of thefinishing line, the steel strand S itself, or thin slabs D divided offfrom it by means of a dividing-off device 4, enter a soaking furnace 5.

In the soaking furnace 5, which is designed as a tunnel furnace, thesteel strand S or the thin slab D is/are heated to a reheat temperatureT_(w) which is above the A_(r3) temperature, but below 1100° C. Theposition of the A_(r3) temperature is also shown in FIG. 2, depending onthe carbon content of the steel composition.

The temperature T_(w) attained during reheating in the soaking furnace 5depends on the annealing treatment which is carried out during furtherprocessing of the hot strip from the steel strand S or the thin slabs D,to form cold strip. If the cold strip which is cold rolled from the hotstrip is subjected to annealing in a hood-type annealing furnace, thenthe reheat temperature T_(w) is in the region of 1100° C. By contrast,if the cold strip after cold rolling is subjected to continuousannealing, then the reheat temperature T_(w) is approx. 1000 C.

FIG. 3 shows the temperature gradient of the steel strand S in themethod according to the invention in a solid line. In addition, indashed lines, it shows the gradient of the steel strand S which occurswith the method according to the state of the art. It is clearly evidentthat with conventional processing of a strand from a low-carbonlow-alloyed steel, the cooling rate a_(LMSdT) in the region of themetallurgical length LM is significantly lower than is the case of themethod according to the invention; that the temperature does not fallbelow the A_(r3) temperature, and that the upper limit of the reheattemperature T_(wStD) is significantly above the upper limit of reheatingof 1100° C. in the invention.

LIST OF REFERENCE CHARACTERS 1 Permanent casting mould 2 Cooling devices3 Roller table 4 Dividing-off device 5 Soaking furnace D Thin slabs LMMetallurgical length LT Cooling line L Path axis of Diag. 2 S Steelstrand T Temperature axis of Diag. 2 T_(LM) Temperature at the end ofthe metallurgical length LM T_(W) Reheat temperature T_(WStdT) Reheattemperature in the state of the art

What is claimed is:
 1. A method for producing a hot rolled strip withgood deep-drawing characteristics, said method comprising: melting asteel melt comprising in % by weight: C: ≦ 0.07%  Si: ≦ 0.5% Mn: ≦ 2.5%Al: ≦ 0.1% N: ≦ 0.01%  P: ≦ 0.025%  B: ≦ 0.05% 

up to a total of 0.35% of Nb, Ti and V, balance iron and inevitableimpurities; continually outputting the steel melt from a permanentcasting mould to form a strand; leading the cast strand along a coolingline immediately after discharging from the permanent casting mould;intensively cooling down the cast strand to a temperature of A_(r1)±25 Kat a cooling rate a_(LM) of at least 3K/s; cooling the cast strand byexposure to air for at least 30 seconds following the intensivelycooling down step; and reheating in a soaking furnace the strand or thinslabs divided off the strand followed by hot rolling the strand or thethin slabs to form hot rolled strips.
 2. The method of claim 1 furthercomprising reheating the strand or the thin slab in the soaking furnaceto a temperature above A_(r3) temperature, but not exceeding 1100° C. 3.The method of claim 1, wherein the thin slabs measure between 20 and 70mm in thickness.
 4. The method of claim 1 further comprising makingseveral hot roll passes during hot rolling, wherein a finish-rolled hotstrip is 2 to 5 mm in thickness.
 5. The method of claim 4, furthercomprising attaining a thickness reduction of the finish-rolled hotstrip of ε_(h)>15% in the last roll pass of the hot rolling step.
 6. Themethod of claim 4, wherein a finish-roll temperature on completion ofthe hot rolling step is at least 20° C. above the A_(r3) temperature. 7.The method of claim 4, wherein the finish-roll temperature on completionof the hot rolling step is below the A_(r1) temperature +50° C.
 8. Themethod of claim 4, further comprising cold rolling the hot rolled stripto make a cold rolled strip from the hot rolled strip, wherein the totaldeformation ε_(ges), achieved during the cold rolling step is at least60%.
 9. The method of claim 8 further comprising annealing the coldrolled strip in a continuous furnace wherein the temperature duringheating the strand or the thin slabs in the soaking furnace does notexceed 1050° C.
 10. The method of claim 4, further comprising coilingthe finish-rolled hot strip at a coiling temperature of at least 650° C.11. The method of claim 8, further comprising annealing the cold rolledstrip in a hood annealing furnace wherein the temperature during heatingthe strand or the thin slabs in the soaking furnace ranges from 1100° C.to 1150° C.
 12. The method of claim 4, further comprising coiling thefinish-rolled hot strip at a coiling temperature of 625C maximum.